The present invention relates to a high performance X-ray generating target. More particularly, the invention is directed to an X-ray anode that resists target substrate debonding.
X-rays are produced when electrons are released in a vacuum within an X-ray tube, accelerated and then abruptly stopped. The electrons are initially released from a heated, incandescent filament. A high voltage between an anode and cathode accelerates the electrons and causes them to impinge upon the anode. The anode, usually referred to as the target, can be a rotating disc type so that the electron beam constantly strikes a different point on the target surface. Typically, a rotating target is made up of a focal track that is bonded to a metal substrate along an interface. The substrate is bonded to a graphite ring. The incidence of high-energy electrons generates large amounts of heat. Unless quickly extracted, the heat can damage the focal track. The metal substrate removes heat away from the focal track and into the graphite ring, which acts a heat sink. The heat is removed from the graphite ring into the surrounding environment.
The X-ray tube contains both the anode assembly and a cathode assembly. The anode assembly includes the rotating disk target and a rotor that is part of a motor assembly that spins the target. A stator is provided outside the X-ray tube vacuum envelope, overlapping about two-thirds of the rotor. The X-ray tube is enclosed in a protective casing having a window for the X-rays that are generated to escape the tube. The casing is filled with oil to absorb heat produced by the X-rays.
Typically the substrate is a refractory metal and the target focal track is an X-ray emitting metal. Tungsten alone and tungsten alloyed with other metals are commonly used in X-ray targets. Metals, which are sometimes alloyed with the tungsten in small amounts, include rhenium, osmium, irridium, platinum, technetium, ruthenium, rhodium and palladium. X-ray targets formed wholly from tungsten or from tungsten alloys where tungsten is the predominant metal are characterized by high density and weight. Additionally, tungsten is notch sensitive and extremely brittle and is thereby subject to catastrophic failure. Because of these shortcomings, X-ray targets typically comprise a tungsten or tungsten alloy target focal track and a target substrate of another metal or alloy. Typically, molybdenum and molybdenum alloy are used for the target substrate.
The target focal track and the target substrate can have different coefficients of thermal expansion (CTE's). For example, a molybdenum or molybdenum alloy substrate can have a higher coefficient of thermal expansion than either the focal track or the graphite backing. The molybdenum or molybdenum alloy substrate expands more than either the tungsten or graphite when subjected to a heating cycle. Thus during tube operation, high stresses are generated at the focal track/substrate interface and at the substrate/graphite interface. Unequal thermal expansion of the target focal track, target substrate and graphite backing coupled with centrifugal force during operation imparts a bending moment to the substrate that tends to move the outer edge of the substrate away from its graphite ring. The target substrate can crack or otherwise weaken and debond from the graphite ring. Thus, there is a need for an X-ray target that resists debonding at the target substrate/target graphite ring interface.